402 Prof. K. A. von Zittel—On the Mammalia. 
triconodont molars, in which the crown is not sharply separated from 
the root, which is simple, or only incompletely-divided by a lateral 
furrow. The slender styliform incisors and powerful canines show 
further that the Triassic Protodonts of America possessed a not less 
differentiated dentition than the Tritylodontide from Europe and 
South Africa. 
In the Jurassic period both the Allotheria and the insect-eating 
Marsupials increased and became further developed. The Great 
Oolite of Stonesfield and the Purbeck ‘ Dirt-bed’ are still the only 
localities in Europe in which they have been found, but besides 
these the Upper Jurassic ‘“ Atlantosaurus Beds” in Wyoming and 
Colorado have yielded a wealth of new forms, which, however, like 
their European contemporaries, are mostly represented by jaws and 
small detached teeth, and very seldom by other bones of the skeleton. 
The Jurassic Allotheria are divided into two families, as yet very 
insufficiently characterized ; of these, one (Bolodontide) is only re- 
presented by upper jaws; the other (Plagiaulacide) principally 
by lower jaws. In the structure of the molars these last agree 
with the Triassic Microlestes, but the lower premolars possess the 
form of cutting, laterally-grooved, blades, like those now only 
present in the grass-eating Kangaroo-rats (Hypsiprymnide). The 
rodent-like incisors also admit of comparison with those of Hypsi- 
prymnus and other Diprotodont Marsupials. The circumstance is 
of interest that up to the present only two genera (Plagiaulax and 
Bolodon) are known from the Purbeck beds of England, and that 
North America possesses two nearly related representative forms in 
Cienacodon and Allodon. Of the minute Stereognathus from the 
Great Oolite of Stonesfield with crescent-shaped tubercles on the 
molars, only a single jaw is known. 
Besides the Allotheria, there are in the Jurassic strata of Hurope 
and North America a considerable number of genera having molars 
with three pointed cusps, conical canines, and styliform or spatulate 
incisors, which seldom exceeded a rat in size and mainly lived on 
insects. The European forms were all placed by Owen with the 
Polyprotodont Marsupials and compared with the living Myrmecobius, 
but it is only in certain genera in which the dentition and form of 
the jaw exhibit such plainly marked marsupial characters, that their 
position can be certainly determined ; in many instances a mingling 
of the peculiarities of Marsupials and Insectivora renders a decision 
impossible. Marsh solved the question in a radical manner by 
placing the Mesozoic mammals with pointed teeth in an indepen- 
dent order, Pantotheria; Osborn recognises in them the ancestors 
of the Polyprotodont Marsupials and of the Insectivora. Unfor- 
tunately our knowledge of these ancient mammals depends at present 
entirely on jaws, isolated teeth, vertebrae, and some rare bones of 
the extremities. No pelvis with the characteristic marsupial bones 
has, as yet, been discovered. As in the Allotheria, so also in the 
Jurassic Polyprotodonts there is an astonishing similarity, and in 
some instances, even complete resemblance between the Huropean 
and American genera. In the following list the corresponding 
